Method of changing reflectance or resistance of a region in an optoelectronic memory device

ABSTRACT

A method for changing reflectance or resistance of a region in an optoelectronic memory device. Changing the reflectance of the region includes sending an electric current through the region to cause a reflectance change in the region. Changing the resistance of the region includes: projecting a laser beam at a first beam intensity on the region, resulting in the region changing from a first to a second different resistance value; electrically reading the second resistance value during which an optical signal carried by the laser beam has a first digital value; after electrically reading the second resistance value, the laser beam is projected at a second beam intensity on the region resulting in the region changing from the second to the first resistance value; and electrically reading the first resistance value of the region while the laser beam is being projected on the region at the second beam intensity.

This application is a continuation application claiming priority to Ser.No. 13/558,541, filed Jul. 26, 2012, now abandoned, which is acontinuation of Ser. No. 12/842,158, filed Jul. 23, 2010, now U.S. Pat.No. 8,288,747 issued Oct. 16, 2012, which is a divisional applicationclaiming priority to Ser. No. 11/161,941, filed Aug. 23, 2005, now U.S.Pat. No. 7,768,815 issued Aug. 3, 2010.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to memory devices, and more specifically,to optoelectronic memory devices.

2. Related Art

A memory cell in a typical semiconductor memory device usually comprisesone or more transistors and can store one of two possible valuesdepending on the voltage potential of a certain node of the memory cell.For instance, the memory cell can be considered storing a 1 if the nodeis at 5V and storing a 0 if the node is at 0V. As a result, the memorycell is an electronic memory cell. In order to write the electronicmemory cell, an appropriate voltage potential is applied to the node (oranother node) of the electronic memory cell. In order to read thecontent of the electronic memory cell, the voltage potential of the node(or another node) of the electronic memory cell can be sensed and thenamplified by a sensor-amplifier (sense-amp) circuit. However, electricaldigital signal transmission is usually slower than optical digitalsignal propagation. Therefore, it would speed up the write and readcycles of the memory cell if either or both of the write and read cyclescan be performed optically. As a result, there is a need for anoptoelectronic memory device that (a) can be written optically (i.e., bylight) or electrically (by applying voltage) and/or (b) can be readoptically (i.e., by light) or electrically (by sensing voltage).

SUMMARY OF THE INVENTION

The present invention provides a method, comprising providing aresistive/reflective region on a substrate, wherein theresistive/reflective region comprises a material having a characteristicof changing the material's reflectance due to a phase change in thematerial; sending an electric current through the resistive/reflectiveregion so as to cause a reflectance change in the resistive/reflectiveregion from a first reflectance value to a second reflectance valuedifferent from the first reflectance value; and optically reading thereflectance change in the resistive/reflective region.

The present invention also provides a method, comprising providing aresistive/reflective region on a substrate, wherein theresistive/reflective region comprises a material having a characteristicof changing the material's resistance due to the material absorbingheat; projecting a laser beam on the resistive/reflective region so asto cause a resistance change in the resistive/reflective region from afirst resistance value to a second resistance value different from thefirst resistance value; and electrically reading the resistance changein the resistive/reflective region.

The present invention also provides a structure, comprising (a) Nregular resistive/reflective regions on a substrate, N being a positiveinteger, wherein the N regular resistive/reflective regions comprise amaterial having a characteristic of changing the material's resistanceand reflectance due to the material absorbing heat; (b) N sense-ampcircuits electrically coupled one-to-one to the N regularresistive/reflective regions, wherein each sense-amp circuit of the Nsense-amp circuits is adapted for recognizing a resistance change in theassociated regular resistive/reflective region; and (c) a lightsource/light detecting device optically coupled to the N regularresistive/reflective regions, wherein the light source/light detectingdevice is adapted for recognizing a reflectance change in each regularresistive/reflective region of the N regular resistive/reflectiveregions.

The present invention provides an optoelectronic memory device that (a)can be written optically (i.e., by light) or electrically (by applyingvoltage) and/or (b) can be read optically (i.e., by light) orelectrically (by sensing voltage).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optoelectronic memory device, in accordance withembodiments of the present invention.

FIG. 2 illustrates one embodiment of optoelectronic memory cells of theoptoelectronic memory device of FIG. 1, in accordance with embodimentsof the present invention.

FIG. 3 illustrates how an applied voltage pulse affects the resistanceof the optoelectronic memory cell of FIG. 2, in accordance withembodiments of the present invention.

FIGS. 4A and 4B illustrate how an applied voltage affects the resistanceof the optoelectronic memory cell of FIG. 2, when the material of theresistive/reflective region has a positive temperature coefficient ofresistance (TCR) and a negative TCR, respectively, in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an optoelectronic memory device 100, in accordancewith embodiments of the present invention. More specifically, theoptoelectronic memory device 100 comprises, illustratively, twooptoelectronic memory cells (OEMC) 110 a and 110 r. The optoelectronicmemory device 100 further comprises a switch 120 for applying voltagepotentials Vcc and V_(WR) to the OEMC 110 a and 110 r in a mannercontrolled by the control signals Va and Vr (further details arediscussed below).

The optoelectronic memory device 100 further comprises a switch 130, asensor-amplifier (sense-amp) circuit 140, and a light source/lightdetecting device 150. The switch 130 electrically couples the OEMC 110 aand 110 r to the sense-amp circuit 140 in a manner controlled by thecontrol signal Vr, whereas the light source/light detecting device 150is optically coupled to the OEMC 110 a and 110 r (further details arediscussed below).

FIG. 2 illustrates one embodiment of the OEMC 100 a of FIG. 1, inaccordance with embodiments of the present invention. More specifically,the OEMC 100 a comprises a resistive/reflective region 210 (comprisingtantalum nitride TaN in one embodiment) embedded in a dielectric layer240. The dielectric layer 240 (comprising silicon dioxide SiO₂ in oneembodiment) is formed on a semiconductor (e.g., silicon) layer 250.

In one embodiment, the TaN resistive/reflective region 210 iselectrically coupled to two electrically conducting lines 220 a and 220b through two vias 230 a and 230 b, respectively. Illustratively, theelectrically conducting lines 220 a and 220 b comprise aluminum (Al) orCopper (Cu) or any other metals, whereas the vias 230 a and 230 bcomprise tungsten (W) or Cu or any other metals. In one embodiment, theAl line 230 a of the OEMC 100 a is coupled to the switch 120 (FIG. 1)whereas the Al line 230 b of the OEMC 100 a is coupled to ground (FIG.1).

In one embodiment, the fabrication of the OEMC 100 a can start out withthe silicon layer 250. Next, in one embodiment, a SiO₂ layer 240 a (thelower portion of the silicon dioxide layer 240) is formed on top of thesilicon layer 250 by, illustratively, CVD (chemical vapor deposition) ofSiO₂.

Next, in one embodiment, the TaN resistive/reflective region 210 isformed on top of the SiO₂ layer 240 a. Illustratively, the TaNresistive/reflective region 210 is formed on top of the SiO₂ layer 240 aby (i) blanket depositing a TaN layer (not shown) on top of the SiO₂layer 240 a and then (ii) directionally and selectively etching thedeposited TaN layer such that what remains of the deposited TaN layer isthe TaN resistive/reflective region 210.

Next, in one embodiment, a SiO₂ layer 240 b (the upper portion of thesilicon dioxide layer 240) is formed on top of the SiO₂ layer 240 a andthe TaN resistive/reflective region 210 by illustratively CVD of SiO₂.As a result, the TaN resistive/reflective region 210 is embedded in theSiO₂ layer 240.

Next, in one embodiment, the two W vias 230 a and 230 b are formed (i)in the SiO₂ layer 240 b and (ii) in electrical contact with the TaNresistive/reflective region 210. Illustratively, the two W vias 230 aand 230 b are formed by (a) creating two holes 230 a and 230 b by anyconventional lithographic process such that the TaN resistive/reflectiveregion 210 is exposed to the surrounding ambient via the holes 230 a and230 b, then (b) blanket depositing a tungsten layer (not shown) so as tofill the two holes 230 a and 230 b with tungsten, and then (c)planarizing the deposited tungsten layer until a top surface 242 of theSiO₂ layer 240 b is exposed to the surrounding ambient.

Next, in one embodiment, the two Al lines 220 a and 220 b are formed (i)on top of SiO₂ layer 240 b and (ii) in electrical contact with the two Wvias 230 a and 230 b, respectively. Illustratively, the two Al lines 220a and 220 b are formed by (a) blanket depositing an Al layer (not shown)on top of the SiO₂ layer 240 b and the two W vias 230 a and 230 b, andthen (b) directionally and selectively etching the deposited Al layersuch that what remain of the deposited Al layer are the two Al lines 220a and 220 b.

In one embodiment, the structure of the OEMC 110 r is similar to thestructure of the OEMC 110 a described above. Moreover, the OEMC 110 r iscoupled to the switch 120 and ground (FIG. 1) in a manner similar to themanner in which the OEMC 110 a is coupled to the switch 120 and ground(FIG. 1).

FIG. 3 shows a plot 300 illustrating how an applied voltage pulseaffects the resistance of the TaN resistive/reflective region 210 of theoptoelectronic memory cell 110 a of FIG. 2, in accordance withembodiments of the present invention. More specifically, the inventorsof the present invention have found that a voltage pulse applied acrossthe TaN resistive/reflective region 210 (FIG. 2) changes both theresistance and the reflectance of the TaN resistive/reflective region210 (FIG. 2). The reflectance is defined as the ratio of the photon fluxreflected by a surface to the photon flux incident on the surface. Inone embodiment, the applied voltage pulse can have a triangular shape.More specifically, the applied voltage pulse comprises an increase from0V to a peak voltage and then a voltage drop back to 0V.

The inventors of the present invention have found that, for a particularsize of the TaN resistive/reflective region 210 (FIG. 2), if the peakvoltage of the applied voltage pulse is between V1 and V2 (for example,V1 and V2 can be 3V and 4V, respectively, represented by segment B-C ofthe plot 300 is applicable), then both the resistance and thereflectance of the TaN resistive/reflective region 210 (FIG. 2) changeas a result of the applied voltage pulse. More specifically, in thissegment B-C of the plot 300, the higher the peak voltage of the appliedvoltage pulse, the higher the resulting resistance and the reflectanceof the TaN resistive/reflective region 210 (FIG. 2). For example, assumethe resistance of the TaN resistive/reflective region 210 (FIG. 2) isoriginally R₀, and that a voltage pulse having a peak voltage of V2(i.e., 4V in the example above) is applied across the TaNresistive/reflective region 210 (FIG. 2). After the pulse is removed,the resistance of the TaN resistive/reflective region 210 (FIG. 2) isR₁. Also after the pulse is removed, the TaN resistive/reflective region210 (FIG. 2) has a higher reflectance.

It should be noted that the change in resistance and reflectance of theTaN resistive/reflective region 210 (FIG. 2) in the example above withrespect to segment B-C of the plot 300 is irreversible. Forillustration, assume that after the pulse described above is removed,another voltage pulse having a peak voltage of V1 (i.e., 3V in theexample above) is applied across the TaN resistive/reflective region 210(FIG. 2). The resistance and reflectance of the TaN resistive/reflectiveregion 210 (FIG. 2) would not change back to original values, but remainessentially unchanged (i.e., R₁ for resistance).

FIG. 4A shows a plot 400 illustrating how an applied voltage affects theresistance of the optoelectronic memory cell of FIG. 2, in accordancewith embodiments of the present invention. More specifically, theinventors of the present invention have found that both the resistanceand the reflectance of the TaN resistive/reflective region 210 (FIG. 2)change reversibly in response to the applied voltage between 0V and V1(in one case, V1=3V). For instance, when the applied voltage changesfrom 0V to V1, the resistance of the TaN resistive/reflective region 210(FIG. 2) increases from R₀ to R₃. Also, although not shown, thereflectance of the TaN resistive/reflective region 210 (FIG. 2)increases (i.e., less transparent). However, when the applied voltagechanges from V1 back to 0V, the resistance and reflectance of the TaNresistive/reflective region 210 (FIG. 2) change back to the originalvalues (i.e., reversible).

With reference to FIGS. 1-4A, in one embodiment, the operation of theoptoelectronic memory device 100 is as follows, assuming that the OEMC110 a and 110 r operate in the segment B-C of the plot 300 (FIG. 3).

In one embodiment, the OEMC 110 a can be electrically written. Assumingthat a 1 is to be written into the OEMC 110 a, then Va and Vr can beadjusted such that the switch 120 electrically couples OEMC 110 a tosignal V_(WR) such that a voltage pulse of signal V_(WR) having a peakvoltage of V2 (i.e., 4V in the example above) is applied across the OEMC110 a. As a result of the pulse, the resistance of the TaNresistive/reflective region 210 (FIG. 2) change from R₀ (initialresistance) to and stays at R₁. Also as a result of the pulse, thereflectance of the TaN resistive/reflective region 210 (FIG. 2)increases (and remains high even after the pulse is removed).

In one embodiment, the content of the OEMC 110 a can be electricallyread. More specifically, Va and Vr can be adjusted such that the switch120 electrically couples the OEMCs 110 a and 110 r to Vcc and such thatthe switch 130 electrically couples the OEMCs 110 a and 110 r to thesense-amp circuit 140. Because the resistance of the OEMC 110 a is high(R₁) while the resistance of the OEMC 110 r stays at R₀, the sense-ampcircuit 140 can recognize such a difference (by comparing the voltagedrops across the OEMC 110 a and 110 r) and accordingly generates a 1 atits output Vout, indicating that the OEMC 110 a stores a 1. It should benoted here that the OEMC 110 r is used as a reference memory cell forreading the content of the OEMC 110 a.

In an alternative embodiment, the content of the OEMC 110 a can beoptically read. More specifically, the light source/light detectingdevice 150 can generate identical incident beams 162 a and 162 r (e.g.,lasers) to the OEMCs 110 a and 110 r, respectively, and receives thereflected beams 164 a and 164 r from the OEMCs 110 a and 110 r,respectively. In one embodiment, the incident laser beams 162 a and 162r have 1.3 μm wavelength with a laser pulse duration of 15 ns and with alaser energy in a range of 0.035 μj to 0.095 μj. Because the OEMCs 110 ais more reflective than the OEMC 110 r (as a result of the appliedvoltage pulse during the write cycle described above), the lightsource/light detecting device 150 can recognize the difference in theintensities of the reflected beams 164 a and 164 r from the OEMCs 110 aand 110 r, respectively, and accordingly generates a 1 indicating thatOEMC 110 a stores a 1. It should be noted here that the OEMC 110 r isused as a reference memory cell for reading the content of the OEMC 110a.

The inventors of the present invention have found that an incident beam(e.g., a laser) can have the same effect as a voltage pulse with respectto changing the resistance and reflectance of the TaNresistive/reflective region 210 (FIG. 2). This is because both the laserbeam and the voltage pulse have the same effect of generating heat inthe TaN resistive/reflective region 210 (FIG. 2), resulting in a phasechange in the material of resistive/reflective region 210 (FIG. 2)leading to the change in the resistance and reflectance of the TaNresistive/reflective region 210 (FIG. 2) as described above. In the caseof the voltage pulse, the voltage pulse generates an electric currentthat passes through and hence generates heat in the TaNresistive/reflective region 210 (FIG. 2). In case of the laser, theenergy of the laser transforms into heat in the TaN resistive/reflectiveregion 210 (FIG. 2).

As a result, in an alternative embodiment, instead of being electricallywritten as described above, the OEMC 110 a can be optically written.More specifically, assuming that a 1 is to be written into the OEMC 110a, then the light source/light detecting device 150 can generate theincident beam 162 a (e.g., a laser) at sufficient intensity to the OEMC110 a such that it is as if a voltage pulse with a peak voltage of V2(i.e., 4V in the example above) were applied across the OEMC 110 a. Inone embodiment, the incident laser beams 162 a and 162 r have 1.3 μmwavelength with a laser pulse duration of 15 ns and with a laser energyin a range of 0.6 μj to 1.5 μj. As a result, both the resistance andreflectance of the TaN resistive/reflective region 210 (FIG. 2)increase. This increase in the resistance and reflectance of the TaNresistive/reflective region 210 (FIG. 2) can be subsequently detectedelectrically and optically as described above.

In an alternative embodiment, instead of operating in the segment B-C ofthe plot 300 as described above, the OEMC 110 a operates in the segmentX-Y of the plot 400 (FIG. 4A). Operating in the segment X-Y of the plot400 (FIG. 4A), the OEMC 110 a can simultaneously be written electricallyand read optically, and as a result, can be used to convert anelectrical signal into an optical signal.

More specifically, in one embodiment, when the applied voltage is 0V (a0 for the electrical signal), the TaN resistive/reflective region 210(FIG. 2) of the OEMC 110 a has a first reflectance. The lightsource/light detecting device 150 can detect the same reflectance forboth OEMCs 110 a and 110 r and accordingly generates a 0 for the opticalsignal. When the applied voltage is V1 (a 1 for the electrical signal),the TaN resistive/reflective region 210 (FIG. 2) of the OEMC 110 a has asecond reflectance higher than the first reflectance. The lightsource/light detecting device 150 can detect the reflectance differencebetween the reflectances of the OEMCs 110 a and 110 r and accordinglygenerates a 1 for the optical signal. In other words, the OEMC 110 a canbe used to convert an electrical signal into an optical signal. In oneembodiment, laser wavelengths of 532 nm, 1064 nm, or 1340 nm can be usedfor the incident laser beams 162 a and 162 r used to read the content ofthe OEMC 110 a while the OEMC 110 a operates in the segment X-Y of theplot 400 (FIG. 4A).

Similarly, operating in the segment X-Y of the plot 400 (FIG. 4A), theOEMC 110 a can simultaneously be written optically and readelectrically, and as a result, can be used to convert an optical signalinto an electrical signal. In one embodiment, regarding the incidentlaser beams 162 a and 162 r, the energy of the lasers used for opticallywriting the OEMC 110 a when the OEMC 110 a operates in the segment X-Yof the plot 400 (FIG. 4A) can be higher than the energy of the lasersused for optically reading the OEMC 110 a when the OEMC 110 a operatesin the segment B-C of the plot 300 (FIG. 3) but lower than the energy ofthe lasers used for optically writing the OEMC 110 a when the OEMC 110 aoperates in the segment B-C of the plot 300 (FIG. 3).

More specifically, in one embodiment, when the intensity of incidentlaser beam 162 a is zero (a 0 for the optical signal), the TaNresistive/reflective region 210 (FIG. 2) of the OEMC 110 a has a firstresistance. The sense-amp circuit 140 can detect the same resistance forboth OEMCs 110 a and 110 r and accordingly generates a 0 for theelectrical signal. When the intensity of incident laser beam 162 a is ata higher level (a 1 for the optical signal), the TaNresistive/reflective region 210 (FIG. 2) of the OEMC 110 a has a secondresistance higher than the first resistance. The sense-amp circuit 140can detect the resistance difference between the resistances of theOEMCs 110 a and 110 r and accordingly generates a 1 for the electricalsignal. In other words, the OEMC 110 a can be used to convert an opticalsignal into an electrical signal.

It should be noted that because the changes of the resistance and thereflectance of the TaN resistive/reflective region 210 (FIG. 2) when theOEMC 110 a operates in the segment X-Y of the plot 400 is smaller thanwhen the OEMC 110 a operates in the segment B-C of the plot 300, thesense-amp circuit 140 and the light source/light detecting device 150need to be more sensitive so as to detect small changes of theresistance and the reflectance of the TaN resistive/reflective region210 (FIG. 2).

In summary, operating in the segment B-C of the plot 300, the OEMC 110 acan function as a one-time write optoelectronic memory cell which, afterbe written, can be read many times either electrically or optically. Incontrast, operating in the segment X-Y of the plot 400, the OEMC 110 acan function as an electrical-optical converter for converting back andforth between electrical digital signals and optical digital signals.

In one embodiment, with reference to FIG. 1, the optoelectronic memorydevice 100 can comprise N OEMCs (not shown) essentially identical to theOEMC 110 a each of which can store one bit of information (N is apositive integer). For each of these N OEMCs, there needs to be (i) awrite switch (not shown but similar to the switch 120, (ii) a readswitch (not shown but similar to the switch 130), and (iii) a sense-ampcircuit (not shown but similar to the sense-amp circuit 140). Moreover,each of these N OEMCs is optically coupled to the light source/lightdetecting device 150 in a manner similar to that of the OEMC 110 a. Theoperation of each of these N OEMCs is similar to that of the OEMC 110 a.In one embodiment, all the N OEMCs of the optoelectronic memory device100 share the same reference OEMC 110 r. Alternatively, each of the NOEMCs of the optoelectronic memory device 100 can have its own referenceOEMC.

It should be noted that the description of the embodiments above issufficient such that a person with ordinary skill in the art couldpractice the invention without undue experimentation.

With reference back to FIG. 2, in the embodiments described above, theresistive/reflective region 210 comprises tantalum nitride TaN. Ingeneral, the resistive/reflective region 210 can be a TaN compositestack including multiple layers (not shown). In one embodiment, theresistive/reflective region 210 can be a SiN/TaN/SiO2/SiN compositestack, a SiN/TaN/SiN composite stack, SiN/SiO2/TaN/SiN composite stack,or any other composite stack that includes a TaN core layer. Also, inthe embodiments described above, the dielectric layer 240 comprisessilicon dioxide SiO₂. Alternatively, the dielectric layer 240 cancomprise a low-K material such as SiCOH, SiLK, and polymers, etc.

In the embodiments described above, the resistance of theresistive/reflective region 210 (FIG. 2) increases when it absorbs asmall heat amount that comes from either a voltage source or alow-energy laser. This is the case shown in FIG. 4A when the material ofthe resistive/reflective region 210 (FIG. 2) has a positive temperaturecoefficient of resistance (TCR). Alternatively, the resistance of theresistive/reflective region 210 (FIG. 2) can decrease when it absorbs asmall heat amount that comes from either a voltage source or alow-energy laser. This is the case shown in FIG. 4B when the material ofthe resistive/reflective region 210 (FIG. 2) has a negative TCR.

It should be noted that TaN can have either positive or negative TCRdepending on the TaN film fabrication process. However, whether thematerial of the resistive/reflective region 210 (FIG. 2) has a positiveor negative TCR, the light source/light detecting device 150 canrecognize a difference (if any) in the reflected beams from the OEMCs110 a and 110 r and operate accordingly.

It should also be noted that the voltage values in FIGS. 3, 4A and 4Bare for illustration only. Therefore, the scope of the claims are not inany way restricted to these values.

Similarly, in the embodiments described above, the reflectance of theresistive/reflective region 210 (FIG. 2) increases when theresistive/reflective region 210 (FIG. 2) absorbs a small heat amountthat comes from either a voltage source or a low-energy laser.Alternatively, the reflectance of the resistive/reflective region 210(FIG. 2) can decrease when the resistive/reflective region 210 (FIG. 2)absorbs a small heat amount that comes from either a voltage source or alow-energy laser.

It should also be noted that a resistance increase does not necessarilyoccurs hand-in-hand with a reflectance increase in either reversible orirreversible case. Similarly, a resistance decrease does not necessarilyoccur hand-in-hand with a reflectance decrease in either reversible orirreversible case. For instance, the inventors of the present inventionhave found that for a particular resistive/reflective region 210 (FIG.2), at some heat absorption level, the resistance of the sample 210increases above the original resistance value while the reflectivitygoes below the original reflectance value. But at a certain higher heatabsorption level, the resistance of the sample 210 goes below theoriginal resistance value while the reflectivity goes above the originalreflectance value.

While particular embodiments of the present invention have beendescribed herein for purposes of illustration, many modifications andchanges will become apparent to those skilled in the art. Accordingly,the appended claims are intended to encompass all such modifications andchanges as fall within the true spirit and scope of this invention.

We claim:
 1. A method for changing reflectance of a region in anoptoelectronic memory device, said method comprising: sending anelectric current through the region so as to cause a change in areflectance of the region from a first value of reflectance to a secondvalue of reflectance different from the first value of reflectance, saidregion comprising a material; and optically reading the change in thereflectance of the region, wherein said sending the electric currentthrough the region comprises maintaining a voltage potential across theregion so as to cause the change in the reflectance of the region,wherein the first value of reflectance corresponding to no voltagepotential is maintained across the region, and wherein the second valueof reflectance corresponding to the voltage potential is maintainedacross the region.
 2. The method of claim 1, wherein said voltagepotential is such that the change in the reflectance of the region isreversible.
 3. The method of claim 1, wherein said optically reading thechange in the reflectance of the region is performed while said voltagepotential is being applied across the region.
 4. A method for changingreflectance of a region in an optoelectronic memory device, said methodcomprising: sending an electric current through the region so as tocause a change in a reflectance of the region from a first value ofreflectance to a second value of reflectance different from the firstvalue of reflectance, said region comprising a material; and opticallyreading the change in the reflectance of the region, wherein saidoptically reading the change in the reflectance of the region comprises:sending a first incident laser beam to the region; receiving a firstreflected beam from the region, wherein the first reflected beam is aresult of a reflection of the first incident laser beam off the region;sending a second incident laser beam to another reference region,wherein the another reference region is essentially identical to theregion; receiving a second reflected beam from the another referenceregion wherein the second reflected beam is a result of a reflection ofthe second incident laser beam off the another reference region; andcomparing beam intensities of the first and second reflected beams.
 5. Amethod for changing reflectance of a region in an optoelectronic memorydevice, said method comprising: sending an electric current through theregion so as to cause a change in a reflectance of the region from afirst value of reflectance to a second value of reflectance differentfrom the first value of reflectance, said region comprising a material;and optically reading the change in the reflectance of the region,wherein the change in the reflectance of the region is due to a phasechange in the material.
 6. The method of claim 5, wherein the materialcomprises TaN.
 7. The method of claim 5, wherein said sending theelectric current through the region comprises applying a voltage acrossthe region.
 8. The method of claim 7, wherein the voltage is such thatthe reflectance of the region remains at the second reflectance valueafter the voltage is removed.
 9. The method of claim 7, wherein saidoptically reading the change in the reflectance of the region isperformed after the voltage is removed.